Some previous approaches to acquiring high quality iris and/or face images of moving subjects include freezing the subject motion by using extremely short exposures. In such approaches, the subject may need to be brightly illuminated by a flash in order to obtain a well exposed image. However, such previous approaches may break down over large distances because the flash power used to obtain an acceptable image may become unsafe for the subject's eyes.
Producing a well exposed image without flash illumination, however, may require extending the image exposure, which can degrade the image quality. For example, extending the image exposure can introduce blur (e.g., motion blur for lateral motion in a plane perpendicular to the camera's optical axis) into the image unless the relative motion between the subject and the camera is reduced and/or eliminated.
Image acquisition systems using orthogonal transfer charge-coupled device (OTCCD) sensors can stabilize the projection of a moving subject on to the sensor by shifting the sensor's pixels' potential wells so as to counter the motion. This image projection stabilization can be performed by a control system that optically measures the actual location of the moving subject in the image and shifts the OTCCD array in real time to track the motion of the subject as close as possible. However, as good as such a control system may be able to track the motion of the subject, in practice it can not operate without a tracking error. The tracking error is manifested as the motion blur and degrades the quality of the resulting OTCCD image.